It is well known that electromagnetic radiation can interfere with the functions of electronic equipment such as computers, airplane equipment and medical equipment. By their very nature, mobile phones provide a source of electromagnetic radiation and in many situations it is desirable that mobile phones be used in the vicinity of other electronic equipment without producing interference. For example, it is necessary for a wearer of a hearing aid that the radiation from a mobile phone does not interfere with the hearing aid to such an extent that the audio signals from the mobile phone earpiece cannot be heard by the wearer of the hearing aid. The introduction of digital wireless technologies has further worsened the effects of mobile phone interference in hearing aids. Hearing aids have been developed that are more immune to electromagnetic radiation than hitherto, but this has only alleviated the problem. In order to overcome the problem fully, the radiation reaching the ear of a user needs to be reduced.
There are many products on the market for reducing the electromagnetic radiation reaching the ear of a mobile phone user but the effectiveness of the products varies. Some products come in the form of accessories for the mobile phone, such as neck loops that plug into compatible phones and inductively couple to the telecoil of the hearing aid such that the phone can be kept at a distance from the hearing aid. Other products are incorporated into the design of the mobile phone, such as radiation shields positioned between the ear and the antenna. However, neck loops require users to buy additional electronic devices in order to be able to use the mobile phone, which increases costs, and shields are considered by many to have a detrimental effect on the attractiveness of the mobile phone. Thus, the solutions offered by these products are not satisfactory.
There is evidence that behind the ear hearing aids are more susceptible to interference than in-the-ear hearing aids and custom canal aids, possibly because the smaller aids are worn further away from the antenna of the phone, are shielded by the user's head and may have less gain. Similarly, it has been noticed that in some circumstances flip phones cause less interference compared to standard bar phones since the antenna of a flip phone is typically located further away from the user's ear, when the phone is in use, compared to the antenna of a standard bar phone. However, due to the limited size of modern mobile phones, the circuits of flip phones are often designed such that although the main antenna element is located away from the earpiece, the electric field around the earpiece is still high. In more detail, the Printed Wire Board of the phone (PWB) is often used as a ground plane that cooperates with the main antenna element. Consequently, currents and electric fields are generated in the PWB. The earpiece is typically located near the physical end of the phone, which coincides with the physical end of the PWB and therefore the end of the electrical path of the ground plane provided on the PWB. The end of the electrical path is also the part of the PWB with the highest impedance. Consequently, the point of strongest electric field in the PWB is located in the vicinity of the earpiece even though the main antenna element is located away from the earpiece.
In some countries, regulations issued by governmental organisations force mobile phone manufacturers to ensure that at least a percentage of their phones are hearing aid compatible. For example, in the US, by 18th Feb. 2005, half of all mobile phone handsets must be designed such that the radiation around the earpiece does not exceed specific RF interference levels as defined by an ANSI standard for hearing aid compatibility. The standard sets out maximum allowed values for both the peak electric field emission and the peak magnetic field emission around the earpiece. According to the ANSI standard, both hearing aids and mobile phones are given a rating equal to a number. If the sum of the ratings for a phone and a hearing aid being used together is equal to five or higher, the combination is acceptable for normal use. By 18th Feb. 2005, half of all mobile phones offered by a manufacturer in the US must have a rating of 3 of higher. According to the current standard, for a mobile phone to qualify for a rating of 3, the peak e-field emission levels near the earpiece must be between 41 and 36 dB (V/m)+0.5×AWF and the peak H-field emission levels must be between −14.4 and −9.4 dB (A/m)+0.5 AWF, where AWF is the absolute weighting function. To get a rating of 4, the e-field emission near the earpiece must be less than 36 dB (V/m)+0.5×AWF and the h-field emission must be less than −14.4 dB (A/m)+0.5*AWF. These regulations put additional pressure on manufacturers to manufacture mobile phones that use internal means to reduce the amount of radiation emitted at the location of the earpiece but that still has a design that appeals to customers.
The invention addresses the above and other issues.